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INFORMATIONAL
Network Working Group J. Rosenberg
Request for Comments: 4353 Cisco Systems
Category: Informational February 2006
A Framework for Conferencing with the
Session Initiation Protocol (SIP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Session Initiation Protocol (SIP) supports the initiation,
modification, and termination of media sessions between user agents.
These sessions are managed by SIP dialogs, which represent a SIP
relationship between a pair of user agents. Because dialogs are
between pairs of user agents, SIP's usage for two-party
communications (such as a phone call), is obvious. Communications
sessions with multiple participants, generally known as conferencing,
are more complicated. This document defines a framework for how such
conferencing can occur. This framework describes the overall
architecture, terminology, and protocol components needed for multi-
party conferencing.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Overview of Conferencing Architecture ...........................6
3.1. Usage of URIs ..............................................9
4. Functions of the Elements ......................................10
4.1. Focus .....................................................10
4.2. Conference Policy Server ..................................11
4.3. Mixers ....................................................11
4.4. Conference Notification Service ...........................12
4.5. Participants ..............................................13
4.6. Conference Policy .........................................13
5. Common Operations ..............................................13
5.1. Creating Conferences ......................................13
5.2. Adding Participants .......................................14
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5.3. Removing Participants .....................................15
5.4. Destroying Conferences ....................................15
5.5. Obtaining Membership Information ..........................16
5.6. Adding and Removing Media .................................16
5.7. Conference Announcements and Recordings ...................16
6. Physical Realization ...........................................18
6.1. Centralized Server ........................................18
6.2. Endpoint Server ...........................................19
6.3. Media Server Component ....................................21
6.4. Distributed Mixing ........................................22
6.5. Cascaded Mixers ...........................................24
7. Security Considerations ........................................26
8. Contributors ...................................................26
9. Acknowledgements ...............................................26
10. Informative References ........................................27
1. Introduction
The Session Initiation Protocol (SIP) [1] supports the initiation,
modification, and termination of media sessions between user agents.
These sessions are managed by SIP dialogs, which represent a SIP
relationship between a pair of user agents. Because dialogs are
between pairs of user agents, SIP's usage for two-party
communications (such as a phone call), is obvious. Communications
sessions with multiple participants, however, are more complicated.
SIP can support many models of multi-party communications. One,
referred to as loosely coupled conferences, makes use of multicast
media groups. In the loosely coupled model, there is no signaling
relationship between participants in the conference. There is no
central point of control or conference server. Participation is
gradually learned through control information that is passed as part
of the conference (using the Real Time Control Protocol (RTCP) [2],
for example). Loosely coupled conferences are easily supported in
SIP by using multicast addresses within its session descriptions.
In another model, referred to as fully distributed multiparty
conferencing, each participant maintains a signaling relationship
with the other participants, using SIP. There is no central point of
control; it is completely distributed amongst the participants. This
model is outside the scope of this document.
In another model, sometimes referred to as the tightly coupled
conference, there is a central point of control. Each participant
connects to this central point. It provides a variety of conference
functions, and may possibly perform media mixing functions as well.
Tightly coupled conferences are not directly addressed by RFC 3261,
although basic participation is possible without any additional
protocol support.
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This document presents the overall framework for tightly coupled SIP
conferencing, referred to simply as "conferencing" from this point
forward. This framework presents a general architectural model for
these conferences and presents terminology used to discuss such
conferences. It also discusses the ways in which SIP itself is
involved in conferencing. The aim of the framework is to meet the
general requirements for conferencing that are outlined in [3]. This
specification alludes to non-SIP-specific mechanisms for achieving
several conferencing functions. Those mechanisms are outside the
scope of this specification.
2. Terminology
Conference: Conference is an overused term, which has different
meanings in different contexts. In SIP, a conference is an
instance of a multi-party conversation. Within the context of
this specification, a conference is always a tightly coupled
conference.
Loosely Coupled Conference: A loosely coupled conference is a
conference without coordinated signaling relationships amongst
participants. Loosely coupled conferences frequently use
multicast for distribution of conference memberships.
Tightly Coupled Conference: A tightly coupled conference is a
conference in which a single user agent, referred to as a focus,
maintains a dialog with each participant. The focus plays the
role of the centralized manager of the conference, and is
addressed by a conference URI.
Focus: The focus is a SIP user agent that is addressed by a
conference URI and identifies a conference (recall that a
conference is a unique instance of a multi-party conversation).
The focus maintains a SIP signaling relationship with each
participant in the conference. The focus is responsible for
ensuring, in some way, that each participant receives the media
that make up the conference. The focus also implements conference
policies. The focus is a logical role.
Conference URI: A URI, usually a SIP URI, that identifies the focus
of a conference.
Participant: The software element that connects a user or automata to
a conference. It implements, at a minimum, a SIP user agent, but
may also implement non-SIP-specific mechanisms for additional
functionality.
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Conference State: The state of the conference includes the state of
the focus, the set of participants connected to the conference,
and the state of their respective dialogs.
Conference Notification Service: A conference notification service is
a logical function provided by the focus. The focus can act as a
notifier [4], accepting subscriptions to the conference state, and
notifying subscribers about changes to that state.
Conference Policy Server: A conference policy server is a logical
function that can store and manipulate the conference policy.
This logical function is not specific to SIP, and may not
physically exist. It refers to the component that interfaces a
protocol to the conference policy.
Conference Policy: The complete set of rules governing a particular
conference.
Mixer: A mixer receives a set of media streams of the same type, and
combines their media in a type-specific manner, redistributing the
result to each participant. This includes media transported using
RTP [2]. As a result, the term defined here is a superset of the
mixer concept defined in RFC 3550, since it allows for non-RTP-
based media such as instant messaging sessions [5].
Conference-Unaware Participant: A conference-unaware participant is a
participant in a conference that is not aware that it is actually
in a conference. As far as the UA is concerned, it is a point-to-
point call.
Cascaded Conferencing: A mechanism for group communications in which
a set of conferences are linked by having their focuses interact
in some fashion.
Simplex Cascaded Conferences: a group of conferences that are linked
such that the user agent that represents the focus of one
conference is a conference-unaware participant in another
conference.
Conference-Aware Participant: A conference-aware participant is a
participant in a conference that has learned, through automated
means, that it is in a conference. A conference-aware participant
can use the conference notification service or additional non-
SIP-specific mechanisms for additional functionality.
Conference Server: A conference server is a physical server that
contains, at a minimum, the focus. It may also include a
conference policy server and mixers.
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Mass Invitation: An attempt to add a large number of users into a
conference.
Mass Ejection: An attempt to remove a large number of users from a
conference.
Sidebar: A sidebar appears to the users within the sidebar as a
"conference within the conference". It is a conversation amongst
a subset of the participants to which the remaining participants
are not privy.
Anonymous Participant: An anonymous participant is one that is known
to other participants through the conference notification service,
but whose identity is being withheld.
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3. Overview of Conferencing Architecture
+-----------+
| |
| |
|Participant|
| 4 |
| |
+-----------+
|
|SIP
|Dialog
|4
|
+-----------+ +-----------+ +-----------+
| | | | | |
| | | | | |
|Participant|-----------| Focus |------------|Participant|
| 1 | SIP | | SIP | 3 |
| | Dialog | | Dialog | |
+-----------+ 1 +-----------+ 3 +-----------+
|
|
|SIP
|Dialog
|2
|
+-----------+
| |
| |
|Participant|
| 2 |
| |
+-----------+
Figure 1
The central component (literally) in a SIP conference is the focus.
The focus maintains a SIP signaling relationship with each
participant in the conference. The result is a star topology, as
shown in Figure 1.
The focus is responsible for making sure that the media streams that
constitute the conference are available to the participants in the
conference. It does that through the use of one or more mixers, each
of which combines a number of input media streams to produce one or
more output media streams. The focus uses the media policy to
determine the proper configuration of the mixers.
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The focus has access to the conference policy, an instance of which
exists for each conference. Effectively, the conference policy can
be thought of as a database that describes the way that the
conference should operate. It is the responsibility of the focus to
enforce those policies. Not only does the focus need read access to
the database, but it needs to know when it has changed. Such changes
might result in SIP signaling (for example, the ejection of a user
from the conference using BYE), and those changes that affect the
conference state will require a notification to be sent to
subscribers using the conference notification service.
The conference is represented by a URI that identifies the focus.
Each conference has a unique focus and a unique URI identifying that
focus. Requests to the conference URI are routed to the focus for
that specific conference.
Users usually join the conference by sending an INVITE to the
conference URI. As long as the conference policy allows, the INVITE
is accepted by the focus and the user is brought into the conference.
Users can leave the conference by sending a BYE, as they would in a
normal call.
Similarly, the focus can terminate a dialog with a participant,
should the conference policy change to indicate that the participant
is no longer allowed in the conference. A focus can also initiate an
INVITE to bring a participant into the conference.
The notion of a conference-unaware participant is important in this
framework. A conference-unaware participant does not even know that
the UA it is communicating with happens to be a focus. As far as
it's concerned, the UA appears like any other UA. The focus, of
course, knows that it's a focus, and it performs the tasks needed for
the conference to operate.
Conference-unaware participants have access to a good deal of
functionality. They can join and leave conferences using SIP, and
obtain more advanced features through stimulus signaling, as
discussed in [6]. However, if the participant wishes to explicitly
control aspects of the conference using functional signaling
protocols, the participant must be conference-aware.
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.....................................
. .
. .
. .
. .
. .
. .
. .
. +-----------+ //-----\\ .
. | | || || .
non-SIP . | Conference| \\-----// .
+---------------->| Policy | | | .
| . | Server |----> | | .
| . | | |Conference| .
| . +-----------+ | Policy | .
| . | | .
| . | | .
+-----------+ . +-----------+ | | .
| | . | | \ // .
| | . | | \-----/ .
|Participant|<--------->| Focus | | .
| | SIP . | | | .
| | Dialog . | |<-----------+ .
+-----------+ . |...........| .
^ . | Conference| .
| . |Notification .
+------------>| Service | .
Subscription. +-----------+ .
. .
. .
. .
. .
.....................................
Conference
Functions
Figure 2
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A conference-aware participant is one that has access to advanced
functionality through additional protocol interfaces, which may
include access to the conference policy through non-SIP-specific
mechanisms. A model for this interaction is shown in Figure 2. The
participant can interact with the focus using extensions, such as
REFER, in order to access enhanced call control functions [7]. The
participant can SUBSCRIBE to the conference URI, and be connected to
the conference notification service provided by the focus. Through
this mechanism, it can learn about changes in participants -
effectively, the state of the dialogs and the media.
The participant can communicate with the conference policy server
using some kind of non-SIP-specific mechanism by which it can affect
the conference policy. The conference policy server need not be
available in any particular conference, although there is always a
conference policy.
The interfaces between the focus and the conference policy, and
between the conference policy server and the conference policy are
non-SIP-specific. For the purposes of SIP-based conferencing, they
serve as logical roles involved in a conference, as opposed to
representing a physical decomposition. The separation of these
functions is documented here to encourage clarity in the
requirements. This approach provides individual SIP implementations
the flexibility to compose a conferencing system in a scalable and
robust manner without requiring the complete development of these
interfaces.
3.1. Usage of URIs
It is fundamental to this framework that a conference is uniquely
identified by a URI, and that this URI identifies the focus that is
responsible for the conference. The conference URI is unique, such
that no two conferences have the same conference URI. A conference
URI is always a SIP or SIPS URI.
The conference URI is opaque to any participants that might use it.
There is no way to look at the URI and know for certain whether it
identifies a focus, as opposed to a user or an interface on a PSTN
gateway. This is in line with the general philosophy of URI usage
[8]. However, contextual information surrounding the URI (for
example, SIP header parameters) may indicate that the URI represents
a conference.
When a SIP request is sent to the conference URI, that request is
routed to the focus, and only to the focus. The element or system
that creates the conference URI is responsible for guaranteeing this
property.
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The conference URI can represent a long-lived conference or interest
group, such as "sip:discussion-on-dogs@example.com". The focus
identified by this URI would always exist, and always be managing the
conference for whatever participants are currently joined. Other
conference URIs can represent short-lived conferences, such as an
ad-hoc conference.
Ideally, a conference URI is never constructed or guessed by a user.
Rather, conference URIs are learned through many mechanisms. A
conference URI can be emailed or sent in an instant message. A
conference URI can be linked on a web page. A conference URI can
also be obtained from some non-SIP mechanism.
To determine that a SIP URI does represent a focus, standard
techniques for URI capability discovery can be used. Specifically,
the callee capabilities specification [9] provides the "isfocus"
feature tag to indicate that the UA is acting as focus in this
dialog. Callee capability parameters are also used to indicate that
a focus supports the conference notification service. This is done
by declaring support for the SUBSCRIBE method and the relevant
package(s) in the caller preferences feature parameters associated
with the conference URI.
Other functions in a conference may be represented by URIs. If the
conference policy is exposed through a web application, it is
identified by an HTTP URI. If it is accessed using an explicit
protocol, it is a URI defined for that protocol.
Starting with the conference URI, the URIs for the other logical
entities in the conference can be learned using the conference
notification service.
4. Functions of the Elements
This section gives a more detailed description of the functions
typically implemented in each of the elements.
4.1. Focus
As its name implies, the focus is the center of the conference. All
participants in the conference are connected to it by a SIP dialog.
The focus is responsible for maintaining the dialogs connected to it.
It ensures that the dialogs are connected to a set of participants
who are allowed to participate in the conference, as defined by the
membership policy. The focus also uses SIP to manipulate the media
sessions, in order to make sure each participant obtains all the
media for the conference. To do that, the focus makes use of mixers.
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When a focus receives an INVITE, it checks the conference policy.
The policy might indicate that this participant is not allowed to
join, in which case the call can be rejected. It might indicate that
another participant, acting as a moderator, needs to approve this new
participant. In that case, the INVITE might be parked on a music-
on-hold server, or a 183 response might be sent to indicate progress.
A notification, using the conference notification service, would be
sent to the moderator. The moderator could then allow this new
participant to join, and the focus could then accept the INVITE (or
unpark it from the music-on-hold server). The interpretation of
policy by the focus is, itself, a matter of local policy, and not
subject to standardization.
When it is necessary to remove a SIP participant (with a confirmed
dialog) from a conference, the focus would send a BYE to that
participant to remove the participant. This is often referred to as
"ejecting" a user from the conference, and is called "mass ejection"
when done for many users. Similarly, if it is necessary to add a new
SIP participant to a conference, the focus would send an INVITE
request to that participant. When done for a large number of users,
this is called mass invitation. Finally, if it is necessary to
change the properties of the media of a session (for example to
remove video) for a SIP participant, the focus can update the session
description for that participant by sending a re-INVITE or UPDATE
[15] request with a new offer to that participant.
In many cases, the signaling actions performed by the focus, such as
ejection or addition of a participant, will change the media
composition of the conference. To affect these changes, the focus
interacts with the mixer. Through that interaction, it makes sure
that all valid participants received a copy of the media streams, and
that each participant sends media to an IP address and port on the
mixer that cause it to be appropriately mixed with the other media in
the conference. The means by which the focus interacts with the
mixer are outside the scope of this specification.
4.2. Conference Policy Server
The conference policy server is a logical component of the system.
It represents the interface between clients and the conference policy
that governs the operation of the conference. Clients communicate
with the conference policy server using a non-SIP-specific mechanism.
4.3. Mixers
A mixer is responsible for combining the media streams that make up
the conference, and generating one or more output streams that are
distributed to recipients (which could be participants or other
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mixers). The process of combining media is specific to the media
type, and is directed by the focus, under the guidance of the rules
described in the media policy.
A mixer is not aware of a "conference" as an entity, per se. A mixer
receives media streams as inputs, and based on directions provided by
the focus, generates media streams as outputs. There is no grouping
of media streams beyond the policies that describe the ways in which
the streams are mixed.
A mixer is always under the control of a focus, either directly or
indirectly. The focus is responsible for interpreting the media
policy, and then installing the appropriate rules in the mixer. If
the focus is directly controlling a mixer, the mixer can either be
co-resident with the focus, or can be controlled through some kind of
protocol. If the focus is indirectly controlling a mixer, it
delegates the mixing to the participants, each of which has its own
mixer. This is described in Section 6.4.
4.4. Conference Notification Service
The focus can provide a conference notification service. In this
role, it acts as a notifier, as defined in RFC 3265 [4]. It accepts
subscriptions from clients for the conference URI, and generates
notifications to them as the state of the conference changes.
The state of the conference includes the participants connected to
the focus, and also information about the dialogs associated with
them. As new participants join, this state changes, and is reported
through the notification service. Similarly, when someone leaves,
this state also changes, allowing subscribers to learn about this
fact.
If a participant is anonymous, the conference notification service
will either withhold the identity of a new participant from other
conference participants, or will neglect to inform other conference
participants about the presence of the anonymous participant. The
choice of approach depends on the level of anonymity provided to the
anonymous participant.
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4.5. Participants
A participant in a conference is any SIP user agent that has a dialog
with the focus. This SIP user agent can be a PC application, a SIP
hardphone, or a PSTN gateway. It can also be another focus. A
conference that has a participant that is the focus of another
conference is called a simplex cascaded conference. They can also be
used to provide scalable conferences where there are regional sub-
conferences, each of which is connected to the main conference.
4.6. Conference Policy
The conference policy contains the rules that guide the operation of
the focus. The rules can be simple, such as an access list that
defines the set of allowed participants in a conference. The rules
can also be incredibly complex, specifying time-of-day-based rules on
participation, conditional on the presence of other participants. It
is important to understand that there is no restriction on the type
of rules that can be encapsulated in a conference policy.
The conference policy can be manipulated using web applications or
voice applications. It can also be manipulated with non-SIP-specific
standard or proprietary protocols.
5. Common Operations
There are a large number of ways in which users can interact with a
conference. They can join, leave, set policies, approve members, and
so on. This section is meant as an overview of the major
conferencing operations, summarizing how they operate. More detailed
examples of the SIP mechanisms can be found in [7].
As well as providing an overview of the common conferencing
operations, each of the subsections in this section of the document
provides a description of the SIP mechanism for supporting the
operation. Non-SIP mechanisms are also possible, but not discussed
here.
5.1. Creating Conferences
There are many ways in which a conference can be created. The
creation of a conference actually constructs several elements all at
the same time. It results in the creation of a focus and a
conference policy. It also results in the construction of a
conference URI, which uniquely identifies the focus. Since the
conference URI needs to be unique, the element that creates
conferences is responsible for guaranteeing that uniqueness. This
can be accomplished deterministically (by keeping records of
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conference URIs, or by generating URIs algorithmically), or
probabilistically, (by creating a random URI with sufficiently low
probabilities of collision).
When conference policy is created, it is established with default
rules that are implementation-dependent. If the creator of the
conference wishes to change those rules, they would do so using a
non-SIP mechanism.
SIP can be used to create conferences hosted in a central server by
sending an INVITE to a conferencing application that would
automatically create a new conference and then place a user into it.
Creation of conferences where the focus resides in an endpoint
operates differently. There, the endpoint itself creates the
conference URI, and hands it out to other endpoints that will be the
participants. What differs from case to case is how the endpoint
decides to create a conference.
One important case is the ad-hoc conference described in Section 6.2.
There, an endpoint unilaterally decides to create the conference
based on local policy. The dialogs that were connected to the UA are
migrated to the endpoint-hosted focus, using a re-INVITE or UPDATE to
pass the conference URI to the newly joined participants.
Alternatively, one UA can ask another UA to create an endpoint-hosted
conference. This is accomplished with the SIP Join header [10]. The
UA that receives the Join header in an invitation may need to create
a new conference URI (a new one is not needed if the dialog that is
being joined is already part of a conference). The conference URI is
then handed to the recently joined participants through a re-INVITE
or UPDATE.
5.2. Adding Participants
There are many mechanisms for adding participants to a conference.
In all cases, participant additions can be first party (a user adds
themself) or third party (a user adds another user).
First person additions using SIP are trivially accomplished with a
standard INVITE. A participant can send an INVITE request to the
conference URI, and if the conference policy allows them to join,
they are added to the conference.
If a UA does not know the conference URI, but has learned about a
dialog which is connected to a conference (by using the dialog event
package, for example [11]), the UA can join the conference by using
the Join header to join the dialog.
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Third party additions with SIP are done using REFER [12]. The client
can send a REFER request to the participant, asking them to send an
INVITE request to the conference URI. Additionally, the client can
send a REFER request to the focus, asking it to send an INVITE to the
participant. The latter technique has the benefit of allowing a
client to add a conference-unaware participant that does not support
the REFER method.
5.3. Removing Participants
As with additions, there are several mechanisms for departures.
Removals can also be first person or third person.
First person departures are trivially accomplished by sending a BYE
request to the focus. This terminates the dialog with the focus and
removes the participant from the conference. The focus can also
remove a participant from the conference by sending it a BYE. In
either case, the focus interacts with the mixer to make sure that the
departed participant ceases receiving conference media, and that
media from that participant are no longer mixed into the conference.
Third person departures can also be done using SIP, through the REFER
method.
5.4. Destroying Conferences
Conferences can be destroyed in several ways. Generally, whether
those means are applicable for any particular conference is a
component of the conference policy.
When a conference is destroyed, the conference policy associated with
it is destroyed. Any attempts to read or write the policy results in
a protocol error. Furthermore, the conference URI becomes invalid.
Any attempts to send an INVITE to it, or SUBSCRIBE to it, would
result in a SIP error response.
Typically, if a conference is destroyed while there are still
participants, the focus would send a BYE to those participants before
actually destroying the conference. Similarly, if there were any
users subscribed to the conference notification service, those
subscriptions would be terminated by the server before the actual
destruction.
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There is no explicit means in SIP to destroy a conference. However,
a conference may be destroyed as a by-product of a user leaving the
conference, which can be done with BYE. In particular, if the
conference policy states that the conference is destroyed once the
last user or a specific user leaves, when that user does leave (using
a SIP BYE request), the conference is destroyed.
5.5. Obtaining Membership Information
A participant in a conference will frequently wish to know the set of
other users in the conference. This information can be obtained in
many ways.
The conference notification service allows a conference-aware
participant to subscribe to it, and receive notifications that
contain the list of participants. When a new participant joins or
leaves, subscribers are notified. The conference notification
service also allows a user to do a "fetch" [4] to obtain the current
listing.
5.6. Adding and Removing Media
Each conference is composed of a particular set of media that the
focus is managing. For example, a conference might contain a video
stream and an audio stream. The set of media streams that constitute
the conference can be changed by participants. When the set of media
in the conference change, the focus will need to generate a re-INVITE
to each participant in order to add or remove the media stream to
each participant. When a media stream is being added, a participant
can reject the offered media stream, in which case it will not
receive or contribute to that stream. Rejection of a stream by a
participant does not imply that the stream is no longer part of the
conference, only that the participant is not involved in it.
A SIP re-INVITE can be used by a participant to add or remove a media
stream. This is accomplished using the standard offer/answer
techniques for adding media streams to a session [13]. This will
trigger the focus to generate its own re-INVITEs.
5.7. Conference Announcements and Recordings
Conference announcements and recordings play a key role in many real
conferencing systems. Examples of such features include:
o Asking a user to state their name before joining the conference,
in order to support a roll call
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o Allowing a user to request a roll call, so they can hear who else
is in the conference
o Allowing a user to press some keys on their keypad to record the
conference
o Allowing a user to press some keys on their keypad to be connected
with a human operator
o Allowing a user to press some keys on their keypad to mute or
unmute their line
User 1
+-----------+
| |
| |
|Participant|
| 1 |
| |
+-----------+
|SIP
|Dialog
Conference |1
Policy +---|--------+
User 2 Server | | | Application
+-----------+ +-----------+ | non-SIP *************
| | | | |-------- * *
| | | | | * *
|Participant|-----------| Focus |------------*Participant*
| 2 | SIP | | | SIP * 4 *
| | Dialog | |--+ Dialog * *
+-----------+ 2 +-----------+ 4 *************
|
|
|SIP
|Dialog
|3
|
+-----------+
| |
| |
|Participant|
| 3 |
| |
+-----------+
User 3
Figure 3
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In this framework, these capabilities are modeled as an application
that acts as a participant in the conference. This is shown
pictorially in Figure 3. The conference has four participants.
Three of these participants are end users, and the fourth is the
announcement application.
If the announcement application wishes to play an announcement to all
the conference members (for example, to announce a join), it merely
sends media to the mixer as would any other participant. The
announcement is mixed in with the conversation and played to the
participants.
Similarly, the announcement application can play an announcement to a
specific user by configuring the conference policy so that the media
it generates is only heard by the target user. The application then
generates the desired announcement, and it will be heard only by the
selected recipient.
The announcement application can also receive input from a specific
user through the conference. To do this, it can use the application
interaction framework [6]. This allows it to collect user input,
possibly through keypad stimulus, and to take actions.
6. Physical Realization
In this section, we present several physical instantiations of these
components, to show how these basic functions can be combined to
solve a variety of problems.
6.1. Centralized Server
In the most simplistic realization of this framework, there is a
single physical server in the network, which implements the focus,
the conference policy server, and the mixers. This is the classic
"one box" solution, shown in Figure 4.
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Conference Server
...................................
. .
. +------------+ .
. | Conference | .
. |Notification| .
. | Server | .
. +------------+ .
. +----------+ .
. |Conference| +-----+ .
. | Policy | +-------+ +-----+| .
. | Server | | Focus | |Mixer|+ .
. +----------+ +-------+ +-----+ .
................//.\.....***.......
// \ *** *
// *** * RTP
SIP // *** \ *
// *** \SIP *
// *** RTP \ *
/ ** \ *
+-----------+ +-----------+
|Participant| |Participant|
+-----------+ +-----------+
Figure 4
6.2. Endpoint Server
Another important model is that of a locally-mixed ad-hoc conference.
In this scenario, two users (A and B) are in a regular point-to-point
call. One of the participants (A) decides to conference-in a third
participant, C. To do this, A begins acting as a focus. Its
existing dialog with B becomes the first dialog attached to the
focus. A would re-INVITE B on that dialog, changing its Contact URI
to a new value that identifies the focus. In essence, A "mutates"
from a single-user UA to a focus plus a single user UA, and in the
process of such a mutation, its URI changes. Then, the focus makes
an outbound INVITE to C. When C accepts, it mixes the media from B
and C together, redistributing the results. The mixed media is also
played locally. Figure 5 shows a diagram of this transition.
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B B
+------+ +------+
| | | |
| UA | | UA |
| | | |
+------+ +------+
| . | .
| . | .
| . | .
| . Transition | .
| . ------------> | .
SIP| .RTP SIP| .RTP
| . | .
| . | .
| . | .
| . | .
| . +----------+
+------+ | +------+ | SIP +------+
| | | |Focus | |----------| |
| UA | | |C.Pol.| | | UA |
| | | |Mixers| |..........| |
+------+ | | | | RTP +------+
| +------+ |
A | + | C
| + <..|.......
| + | .
| +------+ | .
| |Parti-| | .
| |cipant| | .
| | | | .
| +------+ | .
+----------+ .
A .
.
Internal
Interface
Figure 5
It is important to note that the external interfaces in this model,
between A and B, and between B and C, are exactly the same to those
that would be used in a centralized server model. User A could also
implement a conference policy and a conference notification service,
allowing the participants to have access to them if they so desired.
Just because the focus is co-resident with a participant does not
mean any aspect of the behaviors and external interfaces will change.
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6.3. Media Server Component
+------------+ +------------+
| App Server| SIP |Conf. Cmpnt.|
| |-------------| |
| Focus | non-SIP | Focus |
| C.Pol |-------------| C.Pol |
| | | Mixers |
|Notification| | |
| | | |
+------------+ +------------+
| \ .. .
| \\ RTP... .
| \\ .. .
| SIP \\ ... .
SIP | \\ ... .RTP
| ..\ .
| ... \\ .
| ... \\ .
| .. \\ .
| ... \\ .
| .. \ .
+-----------+ +-----------+
|Participant| |Participant|
+-----------+ +-----------+
Figure 6
In this model, shown in Figure 6, each conference involves two
centralized servers. One of these servers, referred to as the
"application server" owns and manages the membership and media
policies, and maintains a dialog with each participant. As a result,
it represents the focus seen by all participants in a conference.
However, this server doesn't provide any media support. To perform
the actual media mixing function, it makes use of a second server,
called the "mixing server". This server includes a focus, and
implements a conference policy, but has no conference notification
service. Its conference policy tells it to accept all invitations
from the top-level focus. The focus in the application server uses
third party call control to connect the media streams of each user to
the mixing server, as needed. If the focus in the application server
receives a conference policy control command from a client, it
delegates that to the media server by making the same media policy
control command to it.
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This model allows for the mixing server to be used as a resource for
a variety of different conferencing applications. This is because it
is unaware of conference policy; it is merely a "slave" to the top-
level server, doing whatever it asks.
6.4. Distributed Mixing
In a distributed mixed conference, there is still a centralized
server that implements the focus, conference policy server, and media
policy server. However, there are no centralized mixers. Rather,
there are mixers in each endpoint, along with a conference policy
server. The focus distributes the media by using third party call
control [14] to move a media stream between each participant and each
other participant. As a result, if there are N participants in the
conference, there will be a single dialog between each participant
and the focus, but the session description associated with that
dialog will be constructed to allow media to be distributed amongst
the participants. This is shown in Figure 7.
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+---------+
|Partcpnt |
media | | media
...............| |..................
. | Mixers | .
. |C.Pol.Srv| .
. +---------+ .
. | .
. | .
. | .
. dialog | .
. | .
. | .
. | .
. +---------+ .
. |Cnf.Srvr.| .
. | | .
. | Focus | .
. |C.Pol.Srv| .
. / | | \ .
. / +---------+ \ .
. / \ .
. / \ .
. / dialog \ .
. / \ .
. /dialog \ .
. / \ .
. / \ .
. / \ .
. .
+---------+ +---------+
|Partcpnt | |Partcpnt |
| | | |
| | ......................... | |
| Mixers | | Mixers |
|C.Pol.Srv| media |C.Pol.Srv|
+---------+ +---------+
Figure 7
There are several ways in which the media can be distributed to each
participant for mixing. In a multi-unicast model, each participant
sends a copy of its media to each other participant. In this case,
the session description manages N-1 media streams. In a multicast
model, each participant joins a common multicast group, and each
participant sends a single copy of its media stream to that group.
The underlying multicast infrastructure then distributes the media,
so that each participant gets a copy. In a single-source multicast
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model (SSM), each participant sends its media stream to a central
point, using unicast. The central point then redistributes the media
to all participants using multicast. The focus is responsible for
selecting the modality of media distribution, and for handling any
hybrids that would be necessitated from clients with mixed
capabilities.
When a new participant joins or is added, the focus will perform the
necessary third party call control to distribute the media from the
new participant to all the other participants, and vice versa.
The central conference server also exposes an interface to the
conference policy. Of course, the central conference server cannot
implement any of the media operations or policies directly. Rather,
it would delegate the implementation to each participant. As an
example, if a participant decides to switch the overall conference
mode from "voice activated" to "continuous presence", they would
communicate with the central conference policy server. The
conference policy server, in turn, would communicate with the
conference policy servers that are co-resident with each participant,
using some non-SIP-specific mechanism, and instruct them to use
"continuous presence".
This model requires additional functionality in user agents, which
may or may not be present. The participants, therefore, must be able
to advertise this capability to the focus.
6.5. Cascaded Mixers
In very large conferences, it may not be possible to have a single
mixer that can handle all of the media. A solution to this is to use
cascaded mixers. In this architecture, there is a centralized focus,
but the mixing function is implemented by a multiplicity of mixers,
scattered throughout the network. Each participant is connected to
one, and only one of the mixers. The focus uses some kind of control
protocol to connect the mixers together, so that all of the
participants can hear each other.
This architecture is shown in Figure 8.
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+---------+
+-----------------------| |------------------------+
| ++++++++++++++++++++| |++++++++++++++++++ |
| + +------| Focus |---------+ + |
| + | | | | + |
| + | +-| |--+ | + |
| + | | +---------+ | | + |
| + | | + | | + |
| + | | + | | + |
| + | | + | | + |
| + | | +---------+ | | + |
| + | | | | | | + |
| + | | | Mixer 2 | | | + |
| + | | | | | | + |
| + | | +---------+ | | + |
| + | |... . .... | | + |
| + .|....| . .|.... | + |
| + ...... | | . | ..|... + |
| + ... | | . | | ....+ |
| +---------+ | | +---------+ | | +---------+ |
| | | | | | | | | | | |
| | Mixer 2 | | | | Mixer 3 | | | | Mixer 4 | |
| | | | | | | | | | | |
| +---------+ | | +---------+ | | +---------+ |
| . . | | . . | | . . |
| . . | | .. . | | .. . |
| . . | | . . | | . . |
+---------+ . | +---------+ . | +---------+ . |
| Prtcpnt | . | | Prtcpnt | . | | Prtcpnt | . |
| 1 | . | | 3 | . | | 5 | . |
+---------+ . | +---------+ . | +---------+ . |
. | . | . |
+---------+ +---------+ +---------+
| Prtcpnt | | Prtcpnt | | Prtcpnt |
| 2 | | 4 | | 6 |
+---------+ +---------+ +---------+
------- SIP Dialog
....... Media Flow
+++++++ Control Protocol
Figure 8
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7. Security Considerations
Conferences frequently require security features in order to properly
operate. The conference policy may dictate that only certain
participants can join, or that certain participants can create new
policies. Generally speaking, conference applications are very
concerned about authorization decisions. Having mechanisms for
establishing and enforcing such authorization rules is a central
concept throughout this document.
Of course, authorization rules require authentication. Normal SIP
authentication mechanisms should suffice for the conference
authorization mechanisms described here.
Privacy is an important aspect of conferencing. Users may wish to
join a conference without anyone knowing that they have joined, in
order to silently listen in. In other applications, a participant
may wish to hide only their identity from other participants, but
otherwise let them know of their presence. These functions need to
be provided by the conferencing system.
8. Contributors
This document is the result of discussions amongst the conferencing
design team. The members of this team include:
Alan Johnston
Brian Rosen
Rohan Mahy
Henning Schulzrinne
Orit Levin
Roni Even
Tom Taylor
Petri Koskelainen
Nermeen Ismail
Andy Zmolek
Joerg Ott
Dan Petrie
9. Acknowledgements
The authors would like to thank Mary Barnes, Chris Boulton and Rohan
Mahy for their comments. Thanks to Allison Mankin for her comments
and support of this work.
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10. Informative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[3] Levin, O. and R. Even, "High-Level Requirements for Tightly
Coupled SIP Conferencing", RFC 4245, November 2005.
[4] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] Campbell, B., "The Message Session Relay Protocol", Work In
Progress, October 2004.
[6] Rosenberg, J., "A Framework for Application Interaction in the
Session Initiation Protocol (SIP)", Work In Progress, February
2005.
[7] Johnston, A. and O. Levin, "Session Initiation Protocol (SIP)
Call Control - Conferencing for User Agents", Work in Progress,
February 2005.
[8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[9] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[10] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[11] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[12] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[13] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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[14] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
[15] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Rosenberg Informational [Page 29]
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